On occasion the e2e tests fail as there is an unexpected error while
deleting an RBD image. The particular tests forcefully removes the pool
where the RBD image is stored. Deleting a volume that has been removed
already (or when its parent pool has been wiped), should succeed.
By catching the error that a pool does not exist (anymore), the
provisioner responds to the DeleteVolume request with succes.
Signed-off-by: Niels de Vos <ndevos@redhat.com>